1. Technical Field of the Invention
The present invention relates to digital communication systems, and in particular to systems for deriving a primary and a secondary logical channel from a common physical channel without increasing the bandwidth needed to accommodate the physical channel.
2. Description of Related Art
The purpose of a digital communication system is to enable the exchange of bit-encoded information among various electronic devices located at various places. The functions necessary to accomplish this purpose can be divided into seven groups, each group corresponding to one of the layers of a seven-layer data-communications protocol model adopted by the international standards community, as described in Section 3,1 of Data Networks by Bertsekas and Gallagher (Prentice-Hall, 1987).
The lowest layer of this model, known as the physical layer, encompasses the methods and apparatus needed to move bits from a source to a destination. These methods and apparatus include such things as transmission wires, connectors, and antennas; modulators and demodulators; and the associated electronics and components needed to communicate a bit stream between adjacent nodes in a communication network by means of fiber optics, coaxial cables, parallel conductor transmission lines, wireless radio links, or some combination of these. In this context, the resulting bitstream is called a physical channel.
Once a physical channel is established and a bitstream can be communicated between network nodes, the bits provided by the bitstream can be organized for the benefit of a user or a plurality of users, thereby providing these users with logical channels derived from the raw bit-moving capacity of the physical channel. The functions needed to accomplish this organization are generally encompassed by the higher layers of the seven-layer protocol model mentioned above.
For example, the North American telephone network includes a transmission method and format known as T1-rate service. This service moves bits between network nodes at the rate of 1.536 million bits per second (Mbps). In one use, the full capacity of the T1 physical channel can be employed to provide a single broadband channel for the benefit of a single user, for example to connect a first high-capacity computer server in a first city to a second high-capacity server in a second city. In a different situation, the capacity of the T1-rate physical channel can be subdivided by multiplexing apparats into a twenty-four channels each having a transmission capacity of 64,000 bits per second (64 Kbps). Through functions encompassed by the higher layers of the protocol model, each of these 64 Kbps channels can be configured to support a different digital conversation or application, thereby subdividing the physical channel into a plurality of logical channels.
One of the organizational functions routinely present in a data-communication network is an error-control mechanism intended to provide some degree of protection against transmission errors. Such errors typically arise from the coupling of external disturbances often called noise into the physical channel, and have the undesired effect of altering the logical state of bits as they transmit the physical channel, and thereby altering the logical state of bits delivered by one or more of the logical channels. This error-control mechanism is typically provided by the data-link-control (DLC) functions encompassed by the higher layers of the protocol model.
Under the operation of a standard DLC, a plurality of bits to be communicated are collected and grouped into a data packet. To the beginning of the packet is appended a packet header comprising flag, address, and control fields needed to enable and assist the operation of other network functions. To the end of the packet is appended a packet trailer comprising flag bits and parity bits. Together, the header, packet, and trailer is called a frame. The purpose of the parity bits carried by the frame is to provide a means of detecting the presence of any bit errors introduced into the frame during its transit across the physical channel.
One particular method of generating and processing parity bits is the cyclic redundancy check (CRC), whose operation can be envisioned most clearly as a series of multiplication and division operations among polynomials having modulo-2 coefficients in recognition of their representation of digital bits. In this representation, the contents of a partial frame (i.e., the frame excluding its header flag and its trailer) can be thought of as an N-degree polynomial, where N is the number of bits in the partial frame. This polynomial is divided by a second polynomial known as the CRC generator polynomial. On completion of the division, the resulting remainder is incorporated into the packet trailer as the parity bits, and the frame is passed to the physical channel for transmission.
Upon receipt of the frame, the receiver again computes the polynomial division, and compares the resulting remainder with the received remainder. Transmission errors are indicated by any disagreement between the remainder as conveyed by the received frame and the remainder as re-computed by the receiver.
The polynomial model as well as the limitations and capabilities inherent in CRCs derived from various generator polynomials in widespread commercial use are described more fully by Boudreau, Bergman, and Irvin, in "Performance of a cyclic redundancy check and its interaction with a data scrambler" (IBM Journal of Research and Development, vol. 38, no. 6, November 1994, pp. 651-658). From mathematical results laid out in this paper, it can be shown that current error-protection schemes often provide excess error-control capacity.
This excess error-control capacity arises from practical design constraints. For example, the number of redundancy bits provided by a commercially useful CRC is normally an integral multiple of eight, due to the byte-oriented nature of today's digital communication apparatus. Moreover, in commercial reality most useful CRC generator polynomials are chosen from a small list of accepted industry standards that provide either eight, sixteen, or thirty-two redundancy bits. For this reason, a system architect might select a 32-bit CRC, which might have an abundance of capacity relative to the task at hand, rather than select a 16-bit CRC whose adequacy might be questionable. Thus, the inflexibility of granularity often leads to a wasteful excess of error-control capacity.
In the problem at hand, excess CRC capacity has important commercial considerations that follow from the nature of the DLC of which the CRC is part. In addition to its error-control functions, the DLC typically controls access to the physical transmission medium, and in this sense imposes the logical channel upon the physical channel. In doing so, the DLC also imposes all of the limits inherent in its predetermined frame structure, and in particular thereby constrains transmission efficiency by cementing-in excess CRC capacity.
The DLC's cementing-in of excess CRC capacity has adverse economic consequences to the end-user of the communication system. If the end-user has a need for a small amount of additional transmission capacity, for example to implement a secondary channel for the purpose of carrying network-management information or for extending the reach of an exhausted control field, that user is required to procure additional physical-channel bandwidth from a common carrier, thereby incurring a penalty in complexity and in operating cost. Such problems become particularly acute when the physical channel is provided by a wireless communication network such as a cellular or satellite network, where the limitations imposed by regulatory agencies to conserve the finite electromagnetic spectrum may make the purchase of additional transmission capacity prohibitively expensive.
Thus, there is a need to enable the DLC to recapture excess error-control capacity and apply this recaptured capacity toward providing a secondary logical channel over which the end-user or the common carrier itself may communicate incidental information without increasing the bandwidth needed to accommodate the physical channel, and in this way to conserve spectral resources in a wireless communication system or to minimize wasteful expenditures in a wireline communication system.